Does the one who suffers from obsessive doubts become an infidel if he does not repeat a prayer concerning whose validity he has doubts, or prayers concerning whose obligatory make-up he has doubts, after he has accustomed himself to not repeating them? And does he become an infidel by seeking a fatwa from his mother, who is not knowledgeable, or does she become an infidel by giving him a wrong answer?

There is no cure for obsessive thoughts except to ignore them and not dwell on them. What you are doing by not repeating ablution and prayer is correct. You are not to be blamed for what you are doing because you are doing what Allah Almighty has legislated for you, and Allah wants you to ignore these obsessive thoughts and not surrender to Satan. Unless you have absolute certainty, do not repeat any prayers or ablution. There is no sin upon you in this, and your obsessive thoughts of disbelief are among the demonic whisperings. It is not permissible for your mother to give you a fatwa (definition) without knowledge; rather, she should ask scholars if she does not know the ruling.